The present invention relates generally to a system and method for starting a three phase motor. More specifically, the present invention relates to a system and method for soft starting a three phase motor by reducing the amount of in-rush current to the motor by using an impedance load, such as a power resistor.
At startup, a three phase motor frequently has a high starting torque on the shaft. This high starting torque can damage parts of the motor and other mechanisms connected to the shaft that are subjected to a load during the startup process. For example, a single phase motor can have a first acceleration of the shaft at startup to obtain the operating speed of the motor, while a three phase motor can have a second, higher, acceleration of the shaft at startup to obtain the operating speed of the motor. This dramatic difference in accelerations correlates to higher torque, up to ten times more torque, for a three phase motor and corresponding greater forces on the motor and mechanisms connected to the shaft. In addition to the high starting torque, the startup of a three phase motor also involves a large in-rush or surge of current to the motor. The in-rush of current to the motor can damage the electrical components of the motor and/or system, pose a safety risk for users and cause a drain on the corresponding power grid of the motor.
One example of the use of a three phase motor is in a compressor of a heating, ventilation and air conditioning (HVAC) system. The three phase motor can be used in a single capacity compressor or a multi-capacity compressor. The multi-capacity compressor can have mechanisms or components that are connected to the shaft that impact a stop block during the startup of the motor. Additionally, in some multi-capacity compressors, the different compressor capacity is obtained by reversing the direction of rotation of the shaft of the motor. In other words, rotating the shaft in the opposite direction in some multi-capacity compressors can result in a different compressor capacity. Upon the shaft reversing direction, which involves the rotating shaft of the motor being stopped and then rotated in the opposite direction, the components or mechanism connected to the shaft can impact stop blocks similar to that described above. The impact of the components or mechanism rotating at a high shaft speed on the stop blocks during startup of the motor can damage the components or mechanisms and reduce the useful life of the compressor.
To limit in-rush currents and reduce the starting torque shaft of a three phase motor, a soft starter can be used with the motor. In many regions of the world, electrical codes require the use of soft starters on HVAC systems to limit in-rush current for single and three-phase applications. There are a variety of different devices or systems that can be used to implement a soft starter for a three phase motor. Some of these devices and systems used in a soft starter can include positive and negative temperature coefficient resistors, semiconductors and solid state electronics. These types of soft starters involve complicated electrical circuits and control programs that can be difficult to install and costly to use.
Therefore, what is needed is a simple and cost-effective system and method for soft starting a three phase motor that can reduce the initial acceleration and torque of the motor to limit the impact load on components of a multi-capacity compressor and that can reduce the in-rush current to the motor.
One embodiment of the present invention is directed to a three phase motor for a multi-capacity compressor having a first capacity in response to a first direction of rotation of the motor and a second capacity greater than the first capacity in response to a second direction of rotation of the motor. The motor includes a plurality of windings and a circuit configured to provide an electrical connection between the plurality of windings and a plurality of input lines of a three phase power supply. The circuit having at least one impedance load connected in series with each winding of the plurality of windings upon startup of the three phase motor. The at least one impedance load connected in series with each winding is configured to receive power from an input line of a three phase power supply. The at least one impedance load connected in series with each winding is configured to provide a first amount of current to a corresponding winding of the plurality of windings. The circuit also has switching means. The switching means is configured and disposed to shunt out or remove the at least one impedance load connected in series with each winding for normal operation of the three phase motor upon expiration of a predetermined time period from startup of the three phase motor. The switching means is configured to provide a second amount of current to the plurality of windings.
The circuit further includes rotation control means. The rotation control means is configured and disposed to control the direction of rotation of the motor. The rotation control means has a first configuration to generate the first direction of rotation in the motor and a second configuration to generate the second direction of rotation in the motor. The rotation control means is switchable between the first configuration and the second configuration prior to startup of the three phase motor. The three phase motor further includes a shaft. The shaft is configured to have a first acceleration in response to the first amount of current being provided to the plurality of windings and a second acceleration greater than the first acceleration in response to a second amount of current being provided to the plurality of windings. The first acceleration of the shaft generates a reduced stop impact force on compressor components connected to the shaft during startup of the three phase motor over the second acceleration of the shaft. The stop impact force is generated from the impact of compressor components against a corresponding stop of a multi-capacity compressor as a result of rotation of the shaft.
Another embodiment of the present invention is directed to a method of starting a multi-capacity compressor to reduce impact forces on compressor components. The multi-capacity compressor has a first capacity in response to a first direction of rotation of a compressor motor and a second capacity greater than the first capacity in response to a second direction of rotation of the compressor motor. The method includes the steps of connecting at least one impedance load to at least one winding of the compressor motor using a first electrical path. The at least one impedance load is configured to provide a first amount of current to the at least one winding. The method further includes the step of accelerating a shaft of the multi-capacity compressor to a first speed of rotation corresponding to the first amount of current provided to the at least one winding.
Next, a second electrical path is provided to the at least one winding of the compressor motor. The second electrical path is configured to provide a second amount of current greater than the first amount of current to the at least one winding of the compressor motor. The method also includes the steps of switching from the first electrical path to the second electrical path after a predetermined time period has elapsed and accelerating the shaft of the compressor motor to a second speed of rotation corresponding to the second amount of current provided to the at least one winding. The acceleration of the shaft to the first speed of rotation generating a reduced stop impact force on compressor components connected to the shaft over the acceleration of the shaft to the second speed of rotation, the stop impact force resulting from compressor components impacting a corresponding stop provided on the shaft
A further embodiment of the present invention is directed to a multi-capacity compressor having a first operating capacity and a second operating capacity. The second operating capacity is greater than the first operating capacity. The multi-capacity compressor includes a three phase motor. The three phase motor has a first direction of rotation and a second direction of rotation opposite to the first direction of rotation. The first direction of rotation of the three phase motor generates the first operating capacity in the multi-capacity compressor and the second direction of rotation of the three phase motor generates the second operating capacity in the multi-capacity compressor. The three phase motor also has a plurality of windings. The compressor also has a shaft connected to the three phase motor, which has a first direction of rotation corresponding to the first direction of rotation of the three phase motor and a second direction of rotation corresponding to the second direction of rotation of the three phase motor. The compressor further includes a plurality of eccentric crankpins disposed on the shaft. Each eccentric crankpin is configured and disposed to drive an associated piston.
The compressor additionally includes at least one stop mechanism disposed on the shaft. The at least one stop mechanism is disposed adjacent to an eccentric crankpin of the plurality of eccentric crankpins. Furthermore, the compressor includes an eccentric cam disposed about an eccentric crankpin of the plurality of eccentric crankpins. The eccentric cam has at least one dog configured and disposed to contact said at least one stop mechanism. The eccentric cam and at least one dog have a first position with respect to the at least one stop mechanism in response to the first direction of rotation of the shaft and a second position with respect to the at least one stop mechanism different from the first position in response to the second direction of rotation of the shaft.
A circuit is configured to provide an electrical connection between the plurality of windings and a plurality of input lines of a three phase power supply. The circuit includes at least one impedance load connected in series with each winding of the plurality of windings upon startup of the three phase motor. The at least one impedance load connected in series with each winding being configured to receive power from an input line of a three phase power supply. The at least one impedance load connected in series with each winding is configured to provide a first amount of current to a corresponding winding of the plurality of windings. The circuit also includes a switching mechanism. The switching mechanism is configured and disposed to shunt out or remove the at least one impedance load connected in series with each winding for regular operation of the three phase motor upon expiration of a predetermined time period. The switching mechanism provides a second amount of current to the plurality of windings.
The circuit further includes a rotation control mechanism configured and disposed to control the direction of rotation of the motor. The rotation control mechanism has a first configuration to generate the first direction of rotation in the three phase motor and a second configuration to generate the second direction of rotation in the three phase motor. The rotation control mechanism is switchable between the first configuration and the second configuration. The shaft has a first acceleration during startup of the three phase motor in response to a first amount of current being provided to the plurality of windings and a second acceleration greater than the first acceleration during regular operation of the three phase motor in response to a second amount of current being provided to the plurality of windings. The first acceleration of the shaft generates a reduced stop impact force upon the at least one dog contacting the corresponding at least one stop mechanism in response to a change in the direction of rotation of the three phase motor over the second acceleration of the shaft.
One advantage of the present invention is that a lower starting acceleration of a motor for a compressor is obtained, which reduces the impact force on components of the compressor thereby extending the life of the compressor and permitting a less expensive compressor mechanical.
Another advantage of the present invention is that the in-rush current to the motor is reduced, which can result in energy savings and a more stable power grid.
A further advantage of the present invention is that it can be used on single capacity compressors, in addition to multi-capacity compressors, that are subject to load conditions during startup of the motor.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.